Most of my ideas come from when I read a novel or a comic, but not satisfied with the way things went in those stories, like, I like that character, I don't want her to die, I would love to read more about her, or I hate that guy, the mc is too soft heart, he should just kill him, or why the heck the mc went with the annoying rich guy, the prince like childhood friend is way better, why don't you look at him etc..
That's mostly how I started my stories, I create my own world, my own stories, my own characters because I want to tell a story of a character with that kind of personality, I want to torture that kind of bad guys, I want a story where a caring sweet childhood friend win the girl.
For stories details, I write down the start and the end ( of the whole story, of an arc, of a chapter, of a scene ) I wanted first, put the character at the start, let them to do their own things to lead to the end, then slowly add smaller details like what do they talk about, how do they act, I dont care about the story mood, foreshadow or plotwist..etc.. at this point.
I'm the type that can not think very widely so I need to write everything down first, when I have a closer and clearer look on my own story, the best ideas naturally comes to me, this is when I decide the character should do this, the story should be that, these mobs character can fire up the mood...etc... sometime I need to re write the whole thing because of that.